1. Field of the Invention
The present invention relates to a hardness tester and a hardness test method.
2. Description of Related Art
An indentation hardness test method is widely known in which an indenter, whose projected shape onto a plane is a polygon, is indented on a surface of a material to be tested and its hardness is calculated from a diagonal length of the polygon impression formed on the material. Such a test method is frequently used for evaluation of mechanical characteristics of metal materials.
As is generally known, the Vickers hardness test method uses an indenter of regular quadrangular pyramid shape made of diamond. A hardness of a material sample is calculated using a relation between an average length of two diagonal lengths of an impression of the regular quadrangular pyramid formed on a surface of the sample and load for impressing the indenter onto the sample. The Knoop hardness test method uses an indenter of rectangular pyramid shape made of diamond. A hardness of a material sample is calculated using a relation between length of a longer diagonal of an impression of the rectangular pyramid formed on a sample surface and load for impressing the indenter onto the sample.
A hardness tester for such a test method using an image processing technique with a computer is disclosed (JP H7-218410A, for example). The method in Patent Document 1 captures an image of an impression, formed on a surface of a sample by impressing an indenter thereon, using an imaging means such as a CCD camera connected to a computer, extracts a boundary (dot series data) indicating each side of the impression or corner portions of the impression by applying binary processing, etc. to the captured image data, and calculates hardness from values of coordinates of the extracted boundary or corner portion.
A technique is also disclosed in which an initial image data of a surface of a sample is captured before forming an impression by a CCD camera, an impression image data of the surface is captured after forming an impression by the CCD camera, a differential image data between the initial image data and the impression image data is calculated, a diagonal length is calculated by determining coordinates' values of corner portions of the impression and sample hardness is calculated based on the diagonal length and impress load (JP H9-210893A, for example).
JP Patent No. 3071956 discloses following procedure to determine Vickers hardness. An impression formed on a surface of a sample is imaged by a TV camera and the impression image is digitalized in multi-value gradation. A binary (two-value) image is made by detecting an optimum threshold value for the multi-value digital image and two-dimensional noise reduction is performed for the binary image. After that rough positions of four sides of the impression in the image are determined and boundary points of the impression are detected from the rough position. Four multidimensional regression curves of the boundary points are determined and intersection points of the four regression curves are estimated as vertexes of the impression. An impression area is calculated by determining lengths of two crossing diagonals of the four vertexes based on the positions in the coordinates. Vickers hardness can be calculated by the impression area and an indentation load of an indenter.
There is a concern that, however, the above conventional technique may produce a result with low reliability because the four sides of the impression are estimated in four approximated multidimensional curves and the intersection points of the curves are estimated as vertexes of the impression. Therefore, accuracy for determining the vertexes' positions largely depends on quality of the noise reduction. In addition, the conventional technique does not estimate the impression region itself but calculates based on edge (boundary) regions or vertex regions, and thus errors may be included in the results and it may cause less reliable results.
According to the conventional technique above explained, a position and shape of the impression in the image are predicted. Therefore, in the case where there is a scratch parallel to a profile of the impression region, it may cause misrecognition, resulting in an inaccurate test result. In addition, it is assumed in the process of the conventional technique that there is only one impression in the captured image. Therefore, the technique cannot be applied to an image containing a plurality of test samples.